1. Field of the Invention
The invention relates to a module for a static mixer for a plastic, flowable, composition to be mixed which is critical with respect to the dwell time. The invention refers further to a corresponding mixer and to uses of the mixer.
2. Description of the Prior Art
In the manufacture of shaped parts of hard PVC, PVC in granulate and/or powder form is mixed with lubricants, stabilizers and other additives and the mixture is then further treated in extruders to form a workable composition. During this, the mixture is heated up and compressed to a suitable working pressure. Twin screw extruders are advantageously employed. The intermediate product thus produced is, however, in a state which is still not sufficiently homogeneous for the manufacture of the shaped parts. Therefore, in many plants static mixers are arranged between the extruders and the molding dies or tools. In these mixers, the mixture is homogenized, and indeed--for the protection of the PVC polymers--at low shear velocities. PVC is, a thermoplast which can be worked above 160.degree. C. It is however, not stable thermally, since hydrogen chloride already segregates at 140.degree. C. In the manufacture of window sections of hard PVC, the plastic is processed at temperatures, between 190 and 200.degree. C. At these temperatures the dwell time in the static mixer is rather critical. The dwell time can turn out to be so long at special locations within the static mixer that discolorations and losses in quality arise. As soon as such unfavorable phenomena set in, they increase very rapidly so that the manufacturing process must be interrupted. The static mixers must be cleaned before the process is continued.